1. Field of the Invention
The present invention relates to a method of manufacturing a metal rotor which may be rotated to adjust the capacitance of a trimmer capacitor.
2. Description of Related Art
FIG. 6 is a sectional view of an example of a known trimmer capacitor which is of background interest to the present invention.
The illustrated trimmer capacitor 1 is of a surface mountable type and its principal components are a stator 2, a metal rotor 3, a metal cover 4 and a spring washer 5.
The stator 2 is constituted by a ceramic dielectric material and incorporates stator electrodes 6 and 7 formed in parallel. Terminals 8 and 9 constituted by conductive films are formed on opposite sides of the stator 2 to be electrically connected to the stator electrodes 6 and 7, respectively. Thus, the stator 2 has a symmetrical structure. This is advantageous in that there is no need for paying attention to the direction of the stator 2 when the trimmer capacitor 1 is assembled. In the case shown in FIG. 6, the stator electrode 7 or the terminal 9 associated therewith may be deleted.
The metal rotor 3 is provided on the stator 2. The metal rotor 3 is shown also in FIGS. 7 and 8. FIGS. 7 and 8 show the metal rotor 3 as viewed from the side of an upper surface 10 and a lower surface 11 thereof, respectively.
The metal rotor 3 has the upper surface 10, the lower surface 11 and a circumferential surface 12 which connects them. As clearly shown in FIG. 8, a surface of a rotor electrode 14 is formed by a surface of a step portion 13 which projects in a semicircular configuration from a part of the lower surface 11. Further, a projecting portion 15 substantially equal in height to the step portion 13 is formed also on the lower surface 11 to prevent tilting of the metal rotor 3 as a result of the formation of the step portion 13. On the other hand, as clearly shown in FIG. 7, a driver groove 16 is formed on the upper surface of the metal rotor 3.
The above-described metal rotor 3 is rotatably contained in the metal cover 4. The metal cover 4 has a configuration suitable for rotatably containing the rotor 3 therein and is secured to the stator 2 by being partially engaged with the stator 2. On an upper surface of the metal cover 4, an adjustment hole 17 is provided for exposing the driver groove 16 of the metal rotor 3. The peripheral portion of this adjustment hole 17 is slightly inclined in a conical configuration to exert a spring action on the metal rotor 3. The metal cover 4 is also formed with a terminal 18 which extends to the lower end of the circumferential surface of the stator 2. This terminal 18 may be soldered to the terminal 9 of the stator 2.
A spring washer 5 is disposed between the upper surface 10 of the metal rotor 3 and the peripheral portion of the adjustment hole 17 on the metal cover 4 and is inclined, for example, in a conical configuration to exert a spring action on the metal rotor 3.
In the trimmer capacitor 1 having such a configuration, the spring action provided by the peripheral portion of the adjustment hole 17 on the metal cover 4 and the spring washer 5 urges the metal rotor 3 against the stator 2 to provide stable contact between the metal rotor 3 and the stator 2. This stabilizes the torque of the metal rotor 3 and the capacitance formed between the stator electrode 6 and rotor electrode 14. This capacitance is drawn out through the terminal 8 which is electrically connected to the stator electrode 6 and through the terminal 18 provided on the metal cover which is electrically connected through the spring washer 5 to the metal rotor 3 forming the rotor electrode 14.
The above-described metal rotor 3 is normally manufactured from a copper alloy by means of cold forging or from a zinc alloy by means of casting. However, cold forging or casting requires a metal mold to be exclusively used for the metal rotor 3. Therefore, such an operation not only requires a high initial cost but also incurs costs associated with a reduction of the life of the metal mold because it applies a relatively high load to the metal mold during processing. In addition, a rotor electrode 14 on a metal rotor 3 which has been just obtained using such a process has surface roughness and waviness as large as several tens of .mu.m or more, while the surface roughness and waviness normally required for a metal rotor 3 of a trimmer capacitor 1 is 5 .mu.m or less. The surface roughness and waviness of a metal rotor 3 is preferably as small as possible because the capacitance becomes more stable the smaller the surface roughness and waviness.
Therefore, a metal rotor 3 obtained using cold forging or casting as described above must be subjected to surface grinding using a lapping machine or the like as a post-treatment to reduce the surface roughness and waviness of the rotor electrode 14. However, in such a surface grinding process, the metal rotor 3 must be arranged on the lapping machine or the like at an orientation with the surface of the rotor electrode 14 in contact with the machine. Further, the depth of grinding must be adjusted, which involves relatively complicated and costly operations.
It is therefore an object of the present invention to provide an economical and efficient method of manufacturing a metal rotor for a trimmer capacitor which does not involve cold forging or casting as described above.